During the administration of an intravenous solution, the flow rate of the liquid to the patient must remain below that level which can needlessly injure him. Frequently, the solution includes a medication. An unacceptably high flow rate can produce excessive concentrations of the medication at different points in the patient's body. The medication, not properly diluted, can then act as a toxic chemical and have a destructive effect upon the tissues of the patient's system.
Even where the solution contains no ingredient other than the usual salts and nutrients, its flow rate must also remain at a safe level. Increasing the flow rate beyond that point allows the intravenous solution to significantly thin the patient's blood. The tissues dependent upon the blood stream for their support may not, when required, receive an adequate supply of the biochemicals entrained in the blood stream. Thus, they could undergo significant damage even though the intravenous solution itself contains only benevolent ingredients.
Various components of a system to limit the flow rate of intravenous solutions to a patient received discussion in the patent applications listed above. The system first includes a casette which forms part of the actual conduit transporting the solution to the patient. It has a metering chamber formed partially of an elastomeric membrane but with a predetermined maximum volume. An inlet and an outlet provide a fluid path for the solution into and out of the metering chamber. A cover slip of plastic limits the membrane's expansion as fluid enters the metering chamber.
To control the flow of fluid into and out of the casette's metering chamber, the system also provides a controller which attaches onto the casette. Two valving members, or rods, pass through the cover slip to open and close, sequentially, the casette's inlet and outlet. To do so, each deforms a portion of the elastomeric membrane in the region of the appropriate valve to block the passage of fluid through it.
A cycle of operation begins with the opening of the casette's inlet and the closing of its outlet. Fluid, under the influence of gravity, flows into the metering chamber until the membrane has reached its largest size. At that point, the chamber contains its predetermined maximum volume.
The inlet closes and immediately afterwards the outlet opens. Fluid from the metering chamber then passes along the conduit and to the patient. After the metering chamber has emptied, the outlet closes with the subsequent opening of the inlet to begin a new cycle. The frequency with which the controller provides the cycles of operation determines the rate of flow of solution to a patient. Switches provided on the controller allow the attendant to vary, within limits, the frequency of the operational cycles and, thus, to change the amount of fluid passing to the patient.
The valving rods, in turn, couple to a rocker arm which constitutes the armature of an E-frame electromagnet. The rocker arm has small permanent magnets glued to its ends. The arm pivots about a point located approximately midway between these magnets and on a line passing lengthwise through the middle leg of the E-frame. The rocker arm pivots between two stable configurations. In the first, the magnet at one of the arm's ends contacts one of the E-frame's side legs while the other ends remain spatially separated from each other. When the rocker arm pivots to its other stable configuration, the second magnet makes contact with the other side leg of the E-frame while the first magnet and the first leg no longer do so.
In one of its configuration, the rocker arm extends the outlet valve member to close the casette's outlet while it retracts the inlet valve member to open the inlet. The metering chamber then fills with intravenous solution. In the second position, the rocker arm opens the outlet and closes the inlet. The fluid that previously filled the metering chamber can then flow to the patient.
The couplings between the rocker arm and the valve members display an appreciable degree of springiness. As a result, the rocker arm, as it pivots between its two positions, maintains the closed valve in that configuration while it forces the other valve member to close the second valve. As the rocker arm continues to pivot, the first valve opens but only after the second valve closes. This "make-before-break" sequence of events provides assurance that both valves on the casette cannot remain open at the same time. This avoids any period of time during which fluid could flow through the casette in an uncontrolled manner.
Electronic circuitry within the controller connects to a coil which surrounds the E-frame's middle leg. Current flowing in a first direction causes both side legs of the E-frame to become magnetic poles of the same type; in other words, both will either become North or both will become South poles. The permanent magnets on the rocker arm, however, project different magnetic poles to the E-frame. Thus, the E-frame, by attracting one of the permanent magnets and repelling the other, places the rocker arm in the desired position. Reversing the current, the coil causes the side legs to both become the other type of magnetic pole. The rocker arm consequently, switches between its two stable positions.
The electronic circuitry within the controller translates the selection of the desired flow rate into the appropriate pulses of current to the coil on the E-frame's middle leg. These current transients place the rocker arm in the required positions for the appropriate lengths of time.
The system including the casette and the controller displays many advantages. These includes the minimal weight inhering in the controller itself. Thus, the controller may simply attach to a casette interposed within the usual flow path of the intravenous solution. It needs no table or shelf on which to sit.
Furthermore, the controller undergoes minimal movement in providing the required management of the casette, especially its valves. At the end of the appropriate time intervals, the rocker arm slightly pviots from one of its stable positions to the other to change the casette's valving configurations. Until the end of the following time interval, it need not and, in fact, cannot undergo further motion. Thus, the controller requires minimal energy to function properly. It consequently may use, as a source of its energy, a small battery, such as those seen in transitor radios.
The amount of force required to move the rocker arm, however, determines the energy drain on the battery powering the controller in its normal operations. Facilitating the rocker arm's changes of position portends the further reduction of the controller's energy requirement and the concommittant increase in the life of its battery.
Operating on battery current greatly expands the utility of the controller and casette system. It allows their use under situations not providing access to a constant supply of electricity. Thus, it can function at the scenes of accidents or other traumatic occurrences and also in buildings while a patient moves from one location to another.